Go with the 350, the 400 auto tranny feels sloppy when going through mud and doens't have the torque that the 350 has

 

Having never driven the AT, I'm not sure what torque is like out of the auto, but it's usually better than it is out of the manuals by the nature of the beast. Having said that, my 400 Foreman with 25 Mudlites was hooked to a trailer with maybe 500 lbs of logs in the snow, with the trailer wheels buried. With no weight on the front, spinning was no problem, moving the trailer was.. I put somebody on the front racks (approx 150 lbs), and had to give it some more gas to get it to spin the wheels, however, it did spin 3 out of 4, and the trailer came clear as well. To me, it's about usable power that I can get to.

 

If you are looking for an ATV for primarily thick, gooey, mud riding, I would go with the 350 due to the ability to upgrade to larger size mud tires. The AT has a tough time putting torque to wheels with aggressive oversize mud tires (mud bugs, vamps, outlaws, etc.). If you are looking for primarily a trail machine than I think the AT is a great way to go. It does have good speed and is really nice riding. Two of my buds have ATs and they are extremely happy with them. I wouldn't want one for thick mud riding though.